The present invention relates to a transmission line test system, and more specifically to a transmission line test system in a broadband ISDN where frames of cells are transmitted in the asynchronous transmission mode (ATM) through transmission lines in the synchronous optical network (SONET) to provide an optical fiber communication network protocol.
Lately, with the expansion of data communication, valuable data as well as voice has been transmitted through public line. Therefore, a high quality data transmission in a communication network is being demanded for the future. A broadband ISDN (B-ISDN) has been developed and standardized to be used with various interfaces for practical use as a communication service network capable of transmitting not only voice and low speed data, but such high speed data as animation, etc. In the B-ISDN, unlike an ordinary switch system, information to be transmitted at a different speed such as voice, low speed data, animation, etc. can be processed collectively in an asynchronous transmission mode (ATM) and continuous information, and burst information can also be processed in the same system.
In the ATM network, data in different bands are contained in a fixed length unit, which is called "cell" to be distinguished from an ordinary packet, and then transmitted or switched. In this system, as data are multiplexed in cells without distinguishing line data from packet data, various services can be offered where different transmission speeds are required, thereby the transmission line can be utilized efficiently.
In the ATM communication, user information is divided into a plurality of parts of appropriate length, each being added a header to form a cell. A header part of a cell comprises, for example, a few bytes, and a data part comprises 32-120 bytes. A channel identifier (VCI) is added to a header to identify a source user. The user information is multiplexed after being formed into a cell, and then transmitted or switched in an ATM highway.
A processor of a switching unit specifies, according to a channel identifier, to which buffer in the switch of a switching unit the cell data should be written. Cell data are called "self-routing data" because they are transmitted autonomously in a network. On the receiving side, a necessary cell can be extracted from an ATM highway according to a channel identifier, and user information can be extracted by removing a header.
In the broadband ISDN switch network, a transmission line in the synchronous optical network (SONET) format is adopted as a transmission means as an optical fiber communication network protocol, for example. In the SONET transmission line, a number of ATM cells are grouped and transmitted in a frame.
In the broadband ISDN as described above, however, an efficient transmission line test method has not been established yet to determine whether or not data are transmitted correctly. Besides, there is a problem that a transmission line test must be conducted using ATM cells or SONET-formatted testframes (including a test cell) selected according to the kind of data to be tested.